Thermoforming is a common technique used to form three-dimensional articles, such as trays, cups, signs, refrigerator door liners, and packages. In a typical thermoforming process, a thermoplastic sheet is initially heated to a temperature above the glass transition temperature so that it becomes pliable. The sheet is then shaped within a thermoforming mold and allowed to cool so that it can retain the desired shape. Thereafter, the molded sheet is cut and trimmed to yield the final thermoformed article. One of the general requirements of a successful thermoforming process is the use of thermoplastic polymers (e.g., polyesters) that are capable of retaining a relatively high degree of melt strength when heated. Unfortunately, polymers of this nature tend to be relatively expensive, and can lead to a significant increase in the total cost of the thermoforming process. For this reason, several attempts have been made to use gaseous blowing agents to create “foamed” sheet structures, thereby lowering the density of the material and, in turn, the polymer content. Unfortunately, the processability and tensile properties of the foamed structure are often compromised due to the uncontrolled pore size and distribution.
As such, a need currently exists for an improved polymeric sheet for use in thermoforming processes that can have a reduced amount of polymers, but yet still exhibit good properties.